Lumbar spinal fusion surgeries are performed hundreds of thousands of times each year. When a particular vertebral segment causes pain for a patient, the surgery prevents movement at the particular vertebral segment to decrease the pain. The surgery fuses adjacent vertebral segments together with bone grafts. In order for these grafts to heal, the adjacent vertebral segments must be kept in place.
In order to keep the adjacent vertebral segments in place, pedicle screws and control rods are used to stabilize the vertebral segments. The pedicle screws are screwed into the pedicle of each of the adjacent vertebral segments. A control rod is then passed through the heads of the pedicle screws, preventing movement between the adjacent vertebral segments.
In order to place the pedicle screws, a needle is inserted to a desired location on the pedicle. A guide wire is then passed through the needle to the pedicle. The guide wire may only be inserted a few millimeters into the pedicle. The guide wire is then used to pass various instruments to the desired location. These instruments are used to dilate the opening and drill a guide path. The guide wire is also used to guide the pedicle screw to the desired location so that it can be screwed into the pedicle.
With such a weak purchase of only a few millimeters in the pedicle and with multiple operations which need to be performed over the guide wire, the guide wire will often slip from the desired location. When the wire slips, the surgeon may lose orientation/surgical landmarks and delay access for screw placement. The guide wire may slip multiple times during a single procedure.
In some cases, the unsecured placement of the guide wire results in misplacement of the pedicle screw. Misplacing the pedicle screw could result in significant harm to the patient. FIG. 1 illustrates examples of the placement of lumbar pedicle screws, including an example of the misplacement of a pedicle screw. The two scenarios 101-102 show a vertebral segment 110 and a pedicle screw 140. Both scenarios 101-102 show a top-down, cross-section view of the vertebral segment 110. The vertebral segment 110 shows pedicle 115, a vertebral body 120, and a spinal canal 125. Nerve bundles (not shown) run through the spinal canal 125 between the vertebral bodies along the spine.
The first scenario 101 illustrates pedicle screw 140 placed correctly through the pedicle 115 into the vertebral body 120. The pedicle screw 140 is placed straight through the pedicle 115 without interfering with the spinal canal 125.
The second scenario 102 shows the pedicle screw 140 placed incorrectly. Pedicle screws may be misplaced when a guide wire slips due to a weak purchase on the bony surface of the posterior elements just posterior to the pedicle, either losing the desired entry location to the pedicle or altering the direction of penetration for the pedicle screws. Because of the angle and missed location at which the pedicle screw 140 is placed, the screw passes through a part of the spinal canal 125 and interferes with nerve bundles (not shown) in the spinal canal. Incorrect placement of the pedicle screws can have potentially serious consequences, such as damaging the nerves or weakening the structure of the vertebral segment.
FIG. 2 illustrates the use of pedicle screws in a spinal fusion operation. FIG. 2 shows pedicle screws 240 inserted into vertebral bodies 220 in two views 201 and 202. View 201 shows a dorsal view of the placement of the pedicle screws 240 and the control rods 242 in the vertebral bodies 220. View 202 shows the same placement of the pedicle screws 240 and the control rods 242 in the vertebral bodies 220 from a side angle. Control rods are shown through the heads 244 of the pedicle screws 240, preventing excessive movement between the vertebral bodies.